The present disclosure is directed to burner operation in furnace or boiler applications. In particular, the present disclosure is directed to burner operation during the combustion of a low-volatile solid fuel such as petroleum coke, including in a relatively non-steady operations.
Injection of substantially pure oxygen has been applied to combustion processes in the steel, glass, and cement industries for years. Production and efficiency benefits from selective oxygen injection for those industries is well-known. More recently, uses of oxygen injection in utility boilers has become more prevalent with applications directed towards emissions reductions, efficiency improvements, as well as in the capture of CO2. The injection of oxygen has also been used to demonstrate the combustion of lower rank fuels such as petroleum coke.
The complexity of the burner ranges and may be dependent upon application. In addition, the burners may have multiple adjustments to positional settings. For example, utility boiler air-fuel burners have settings for swirl applied to air, damper positions to balance air flow to multiple burners and nozzle positions adjustments, which may or may not be automated and tied into a control system. Advances in control systems have made it possible to adjust the mentioned variables online, along with traditional adjustments like combustion air fan settings. Oxygen enriched combustion adds another factor in which to control combustion. However, the consequences of mismanaging the oxygen flow supply and distribution can be as severe as damaging the burner or surrounding surfaces. Pure oxygen is expensive and changes in oxygen flow and distribution are frequently made to maintain stability and efficiency of the combustion process.
For example, in some applications oxygen supply would entail a large air separation unit which once built cannot be easily expanded for additional supply. The maximum available oxygen will be substantially fixed. As such, unanticipated large oxygen demands by the combustion system may result in unacceptably low oxygen flow rates at injection points throughout the combustion system. The minimum oxygen supply will be fixed by the turndown rate. In addition oxygen supply rate cannot be adjusted instantaneously in the event of changes in oxygen demand by the combustion system.
Oxygen is typically generated for use by several industrial methods. One of the most common oxygen generation methods is the cryogenic separation of air to produce oxygen (in various purities) and other by-products (typically, nitrogen and argon). The product oxygen can be either gaseous or liquid in form and is usually produced in tonnage quantities (from 25 tons per day of oxygen and up to 5000 tons per day of oxygen). Newer systems may enable even greater production rates of oxygen from a single air separation unit. The liquid oxygen produced (if any) can be stored in liquid tanks for either transport or latter use. A back-up supply of liquid oxygen may also be used in conjunction with an air separation unit. Alternatively, oxygen can be generated via a pressure swing/vacuum swing operation over adsorbent beds. Other processes, such as membrane based systems, can also produce an oxygen predominant stream. Other off-gas streams may be available to provide an oxygen stream. In all the systems, a given amount of oxygen can be supplied at a given time. If more oxygen is required, either time is needed to increase the supply via process conditions or supplement supplies must be accessed. However, it is preferable that the process utilizing the oxygen is not directly affected during this transition. In addition, there are cases where no additional oxygen can be supplied. For example, the oxygen demand is greater than the supply, or there is a bottleneck in the supply, or there is a transport limitation (control valve limit for pipe supply or road limit for truck supply). In all these cases, it is important to maintain the combustion process.
A purpose of oxygen enrichment in combustion applications, particularly those involving high moisture, low volatile or low Btu solid fuels, is the enhancement of flame stability. For applications employing several burners, the continuous optimization of oxygen injection to the various endpoints becomes a significant challenge, and the penalty of non-optimal operation can be substantial. As an example, lower than optimal oxygen enrichment to a burner operating in a steam-generating boiler can result in a loss-of-flame condition that will trigger the shutdown of a mill with subsequent shutdown of an entire row of burners and, hence, an unplanned loss of steam generation. Conversely, higher than desirable oxygen enrichment can dramatically increase NOx emissions and, in a worst-case scenario, can potentially result in high-temperature burner or water-tube failures.
What is needed is a method for combustion system operation and management of support gas flow rates from a substantially fixed source of support gas to a plurality of burners firing in a combustion system, such as a boiler or furnace. In particular, what is further needed is a method and system for combustion of fuel in a relatively non-steady application such as a steam generating boiler used for production of electric power, wherein the supply of oxygen is substantially fixed and may vary.